Analysis of isolated and aperiodic structures with simultaneous multiple angle of incidence measurements

ABSTRACT

A method is disclosed for evaluating isolated and aperiodic structure on a semiconductor sample. A probe beam from a coherent laser source is focused onto the structure in a manner to create a spread of angles incidence. The reflected light is monitored with an array detector. The intensity or polarization state of the reflected beam as a function of radial position within the beam is measured. Each measurement includes both specularly reflected light as well as light that has been scattered from the aperiodic structure into that detection position. The resulting output is evaluated using an aperiodic analysis to determine the geometry of the structure.

PRIORITY CLAIM

[0001] This application claims priority from prior provisionalapplications Serial Nos. 60/355,729, filed Feb. 5, 2002 and 60/356,074,filed Feb. 11, 2002, both of which are incorporated herein by reference.This application is a continuation of U.S. Ser. No. 10/243,245, filedSep. 13, 2002, which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The subject invention relates to optical metrology equipment formeasuring critical dimensions and feature profiles of isolated andaperiodic structures on semiconductor wafers. The invention isimplemented using data obtained from simultaneous multiple angle ofincidence measurements as an input to analytical software designed toevaluate surface features via a scatterometry approach.

BACKGROUND

[0003] There is considerable interest in measuring small geometricalstructures formed on semiconductor wafers. These structures correspondto physical features of the device including conductive lines, holes,vias and trenches as well as alignment or overlay registration markings.These features are typically too small to be measured with conventionaloptical microscopes. Accordingly, optical scatterometry techniques havebeen developed to address this need.

[0004] In a conventional optical scatterometry system, a light beam isdirected to reflect of a periodic structure. The periodic structure actsas an optical grating, scattering some of the light. The light reflectedfrom the sample is then measured. Some systems measure light diffractedinto one or more higher orders. Other systems measure only thespecularly reflected light and then deduce the amount of light scatteredinto higher orders. In any event, the measurements are analyzed usingscattering theory, for example, a Rigorous Coupled Wave Analysis, todetermine the geometry of the periodic structure.

[0005] Rigorous Coupled Wave Theory and other similar techniques relyupon the assumption that the structure which is being inspected isessentially periodic. To match theory to experiment, the diameter of thelight beam spot on the sample is typically significantly larger thanindividual features on the test structure and encompasses many cycles ofthe grating. Most prior art systems operate wherein the probe light beamspot overlaps at least twenty repeating patterns so that the diffractionanalysis will have statistical significance. The results of the analysisrepresent an average of the geometry illuminated by the probe beam.

[0006] In real world semiconductor devices, many (if not most) featuresare isolated or aperiodic. These isolated structures cannot notevaluated with the grating analysis approaches described above.Accordingly, in order to monitor the geometry of isolated featureswithin the dies on the wafer, manufacturers build test structures on the“streets” separating the dies. These test structures are periodic butare intended to have the same geometry (e.g. width, shape) as individualfeatures within the die. By measuring the shape of the tests structures,one can gain information about the structure in the dies or overlayregistration.

[0007] This latter approach has been finding acceptance in the industry.Examples of prior art systems which rely on scatterometry techniques canbe found in U.S. Pat. Nos. 5,867,276; 5,963,329; and 5,739,909. Thesepatents describe using both spectrophotometry and spectroscopicellipsometry to analyze periodic structures and are incorporated hereinby reference. See also PCT publication WO 02/065545, incorporated hereinby reference-which describes using scatterometry techniques to performoverlay metrology.

[0008] In addition to multiple wavelength measurements, multiple anglemeasurements have also been disclosed. In such systems, both thedetector and sample are rotated in order to obtain measurements at bothmultiple angles of incidence and multiple angles of reflection. (See,U.S. Pat. No. 4,710,642)

[0009] About fifteen years ago, the assignee herein developed andcommercialized a multiple angle of incidence measurement system whichdid not require tilting the sample or moving the optics. This system isnow conventionally known as Beam Profile Reflectometry® (BPR®). This andrelated systems are described in the following U.S. Pat. Nos.:4,999,014; 5,042,951; 5,181,080; 5,412,473 and 5,596,411, allincorporated herein by reference. The assignee manufactures a commercialdevice, the Opti-Probe which takes advantage of some of thesesimultaneous, multiple angle of incidence systems. A summary of all ofthe metrology devices found in the Opti-Probe can be found in U.S. Pat.No. 6,278,519, incorporated herein by reference.

[0010] In the BPR tool, a probe beam from a laser is focused with astrong lens so that the rays within the probe beam strike the sample atmultiple angles of incidence. The reflected beam is directed to an arrayphotodetector. The intensity of the reflected beam as a function ofradial position within the beam is measured. Each detector elementcaptures not only the specularly reflected light but also the light thathas been scattered into that detection angle from all of the incidentangles. Thus, the radial positions of the rays in the beam illuminatingthe detector correspond to different angles of incidence on the sampleplus the integrated scattering from all of the angles of incidencecontained in the incident beam. The portion of the detector signalrelated to the specularly reflected light carries information highlyinfluenced by the compositional characteristics of the sample. Theportion of the detector signal related to the scattered light carriesinformation influenced more by the physical geometry of the surface.

[0011] U.S. Pat. No. 5,042,951 describes an ellipsometric version of theBPR, which, in this disclosure will be referred to as Beam ProfileEllipsometry (BPE). The arrangement of the BPE tool is similar to thatdescribed for the BPR tool except that additional polarizers and/oranalyzers are provided. In this arrangement, the change in polarizationstate of the various rays within the probe beam are monitored as afunction of angle of incidence. Both the BPR and BPE tools wereoriginally developed for thin film analysis. One advantage of thesetools for thin film analysis is that the laser beam could be focused toa small spot size on the sample. In particular, the lens can produce aspot of less than five microns in diameter and preferably on the orderof 1 to 2 microns in diameter. This small spot size permittedmeasurements in very small regions on the semiconductor.

[0012] This clear benefit in the thin film measurement field was seen asa detriment in the field of measuring and analyzing gratings with ascatterometry approach. More specifically, a spot size on the order of 1to 2 microns encompasses less than twenty repeating lines of aconventional test grating. It was felt that such a small sampling of thestructure would lead to inaccurate results.

[0013] One approach for dealing with this problem was described in U.S.Pat. No. 5,889,593 incorporated herein by reference. This patentdescribes adding an optical imaging array to the BPR optics whichfunctions to break the coherent light into spatially incoherent lightbundles. This forced incoherence produces a much larger spot size, onthe order of ten microns in diameter. At this spot size, a suitablenumber of repeating features will be measured to allow analysisaccording to a grating theory.

[0014] In U.S. Pat. No. 6,429,943 (incorporated by reference), theinventors herein disclosed some alternate approaches for adapting BPRand BPE to measuring periodic gratings. In one approach, the laser probebeam is scanned with respect to the repeating structure to collectsufficient information to analyze the structure as a grating. In anotherapproach, an incoherent light source is used as the probe beam. Theincoherent source creates a spot size significantly larger than thelaser source and thus could be used to analyze gratings.

[0015] Semiconductor manufacturers continually strive to reduce the sizeof features on a wafer. This size reduction also applies to the width ofthe streets, typically used as the location for the test structuresincluding overlay registration markings. With narrower streets, the sizeof the test structures need to be reduced. Ideally, test structurescould be developed that were not periodic gratings but closer in form tothe actual isolated or aperiodic structures on the dies. Even moredesirable would be to develop an approach which would permit measurementof the actual structures within the dies.

[0016] With today's small feature sizes, it has been generally believedthat direct accurate measurements of isolated or substantially aperiodicstructures could not be performed. An isolated structure wouldcorrespond to, for example, a single line, trench, hole or via or aspecific alignment mark. Such a structure can have extremely smalldimensions (i.e., a single line can have a width of about a tenth of amicron).

[0017] In order to optically inspect such small structures, a very smallillumination spot is desirable. In the broadband applications such asthose discussed above, the probe beam spot size is relatively large, onthe order of 50 microns in diameter. If this probe beam was focused onan isolated structure, the portion of the measured signal attributableto the isolated structure would be extremely small. While the spot sizeof a laser beam is much smaller, it was not envisioned that a enough ofa signal could be obtained to measure an isolated feature. Nonetheless,in initial experiments, it has been shown that BPR and BPE techniquesusing a laser as a probe source can generate meaningful data forisolated structures.

SUMMARY OF THE INVENTION

[0018] In accordance with this invention, an isolated structure (line,via, etc.) is monitored using an illumination source which is coherent,i.e. supplied by a laser. Such a light source can be focused to a probebeam spot size less than five microns in diameter and preferably lessthan two microns in diameter. While even this spot size is much largerthan the feature of interest, that portion of the measured signalattributable to the feature would be much larger than in a broadbandsystem with a much larger spot size.

[0019] The reflected probe beam is monitored with an array detector. Asdescribed in the assignee's patents cited above, when using multipleangle of incidence measurements techniques such as beam profilereflectometry (BPR) and beam profile ellipsometry (BPE), the arraydetector is used to simultaneously generate information as a function ofangle of incidence. The measured data includes a combination ofspecularly reflected light at specific angles of incidence and scatteredlight from all of the angles of incidence. In an alternate embodiment, abaffle element is utilized to block the specularly reflected light andmaximize the scattered light signal.

[0020] The multiple angle of incidence measurements provide informationwhich can be used in a scatterometry analysis. Since the feature isaperiodic, the approach would not take the form of a grating analysisusing Rigorous Coupled Wave Theory. Rather, the analysis would have toconsider light scattered from the isolated structure such as by usingeither a boundary integral or volume integral approach.

[0021] The subject concept is not limited to investigating a singlesmall feature, but rather, is directed in general to investigatingaperiodic structures that cannot be analyzed with a simple gratingmodel.

[0022] For example, consider a single structure whose size is largerthan the probe beam spot. The measured signal could not be analyzed witha grating model, but if sufficient information is known in advance aboutthe structure, it could be analyzed with a boundary integral approach.

[0023] Also consider a periodic structure whose size was smaller thanthe probe beam spot. The measured signal would be the result of anaperiodic illumination field. Such an aperiodic illumination field couldbe analyzed with a spatial averaging or a mixing approach (includingcontributions from both the structure and the surrounding area). Oneexample might be a repeating structure having only 10 lines and wherethe spot was large enough to cover 10 lines (along an axis perpendicularto the longitudinal lines) as well as areas of equal size outside of theline structure.

[0024] The approach can also be used for structures which have some, butincomplete periodicity. For example, consider a line structure havingedge profiles that differ over the structure. Such a structure wouldneed to be analyzed as having an aperiodic geometry.

[0025] It should be understood that by using the appropriate analysis,one can investigate a variety of both periodic and aperiodic structureswithout scanning the probe beam to gain additional information.Nonetheless, it should also be understood that scanning the beam withrespect to the aperiodic structure can provide additional information.Therefore, it is within the scope of the subject invention to evaluateaperiodic structures by scanning the probe beam with respect to thestructure.

[0026] In addition to the structures discussed above, the subjectapproach can also be applied to analysis of the registration ofoverlying patterns created during lithography steps in semiconductormanufacturing.

BRIEF DESCRIPTION OF THE DRAWING

[0027]FIG. 1 is a schematic diagram of an apparatus for performing themethod of the subject invention.

[0028]FIG. 2 is a graph of normalized reflectivity as a function ofangle of incidence and comparing actual measured data to predicted dataassociated with an isolated trench.

[0029]FIG. 3 is a graph of normalized reflectivity as a function ofangle of incidence and comparing actual measured data to predicted dataassociated with an unpatterned thin film.

[0030]FIG. 4 is a cross-section shape of a trench predicted by ananalysis of the measured data illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Turning to FIG. 1, a basic schematic diagram of a simultaneousmultiple angle of incidence apparatus 30 is illustrated. Further detailsabout such a device are described in U.S. Pat. Nos. 4,999,014;5,042,951; 5,159,412; 5,412,473 and 6,429,943, all incorporated hereinby reference. As noted above, the assignee's Opti-Probe deviceincorporates portions of this technology and markets the measurementsubsystem under the trademarks Beam Profile Reflectometry or BPR (aswell as a Beam Profile Ellipsometry (BPE) variant described in U.S. Pat.No. 5,181,080). In the past, the BPR and BPE technologies were utilizedprimarily to analyze the characteristics of thin films and, veryrecently, periodic grating structures formed on semiconductors. Thisdisclosure is directed to using the measurements which can be obtainedfrom this type of system to evaluate the geometry of isolated featuresand aperiodic structures formed on semiconductors.

[0032] The basic measurement system includes a light source 32 forgenerating a probe beam 34. To carry out this method, the light sourceshould be a laser for generating a coherent beam of radiation. Laserdiodes are suitable laser sources for this application. If the output ofthe laser is not itself polarized, a separate linear polarizer can beprovided. The laser output can be coupled to a fiber delivery system asdescribed in PCT WO 01/55671.

[0033] The probe beam 34 is focused onto the feature 12 on the sample 10using a lens 40 in a manner so that the rays within the probe beamcreate a spread of angles of incidence. The sample is shown supported bya stage 44. In the preferred embodiment, the beam is directed normal tothe surface of the sample but can be arranged off-axis as illustrated inU.S. Pat. No. 5,166,752, incorporated by reference. Lens 40 can be ahigh numerical aperture lens (on the order of 0.90 NA) to create anglesof incidence from zero to about 70 degrees. A lens having an NA of atleast 0.5 is preferred. A total range of angles of incidence of at leastthirty degrees is also preferred. The lens creates rays havingpredominantly S-polarized light along one axis and predominantlyP-polarized light along an orthogonal axis. At intermediate angles, thepolarization is mixed.

[0034] In certain measurement situations, it may be desirable to havethe probe beam 34 underfill lens 40 resulting in a lower effectivenumerical aperture. For example, the beam may be configured so that theeffective numerical aperture is 0.5 which would create a spread ofangles of incidence from zero to about 30 degrees. The actual larger NAof the lens (0.9) would be used beneficial in collecting a largerportion of the reflected and scattered light.

[0035] Lens 40 is positioned to create a probe beam spot 42 on thesample. When using a high numerical aperture lens (0.9) that isoverfilled by the probe beam, a spot size having a diameter as small astwo microns or less can be achieved. If a lens with a smaller NA isused, the spot size would typically be larger. In most measurementssituations, it would appear desirable to have the spot size less thanfive microns in diameter. However, in the situation discussed above,where the lens is underfilled, the spot size could be larger than fivemicrons.

[0036] Light reflected by the feature (both specular and scattered) iscollected by the lens 40 and collimated. The reflected light isredirected by a splitter 46 to an imaging lens 48. Lens 48 magnifies andrelays an image of the sample at the focal plane of the lens. A spatialfilter 50 having an aperture is placed in the focal plane of the lens 48for controlling size of the area of the sample which is measured.

[0037] The probe beam is then passed through a 50-50 splitter anddirected to two photodetectors 54 and 56 having a linear array ofdetector elements. The photodetectors are arranged orthogonal to eachother to measure both the S and P polarization components. As describedin detail in the above-cited patents, each of the detecting elements inthe array measures light specularly reflected from different angles ofincidence. The radial position within the reflected probe beam is mappedto the angle of incidence, with the rays closer to the center of thebeam having the smallest angles of incidence and the rays in theradially outer portion of the beam corresponding to the greatest anglesof incidence. Thus, each detector element simultaneously generates anindependent signal that correspond to a different angle of incidence onthe sample plus the integrated scattering from all of the angles ofincidence contained in the incident beam.

[0038] The output signals from the detector arrays are supplied to theprocessor 60. Processor will analyze the signals based on algorithmwhich considers the reflected and scattered light. The algorithms relyon the Fresnel equations. As noted above, since the structure is notperiodic, Rigorous Coupled Wave Theory would not be well suited to theanalysis. Rather, and as discussed below, bounded or volume integralapproaches, are better suited to this problem.

[0039] The selected algorithms will correlate the variation inreflectivity as a function of the position on the detector with thegeometry of the aperiodic structure. The type of analysis will depend onthe application. For example, when used for process control, either insitu or near real time, the processor can compare the detected signalsto an expected set of signals corresponding to the desired geometry ofthe aperiodic structure. If the detected signals do not match theexpected signals, it is an indication that the process is not fallingwithin the specified tolerances and should be terminated andinvestigated. In this approach, no sophisticated real time analysis ofthe signals is necessary

[0040] The reflected output signals can be more rigorously analyzed todetermine the specific geometry of the aperiodic structure. While thereare a number of different approaches, most have certain traits incommon. More specifically, the analytical approach will typically startwith a theoretical “best guess” of the geometry of the measuredstructure. Using Fresnel equations covering both the reflection andscattering of light, calculations are applied to a theoretical model ofthe structure to determine what the expected measured output signalswould be for the theoretical geometry. These theoretical output signalsare compared to the actual measured output signals and the differencesnoted. Based on the differences, the processor will generate a new setof theoretical output signals corresponding to a different theoreticalstructure. Another comparison is made to determine if the theoreticalsignals are closer to the actual measured signals. These generation andcomparison steps are repeated until the differences between thetheoretically generated data and the actually measured data aresubstantially minimized. Once the differences have been minimized, thetheoretical structure corresponding to the best fit theoretical data isassumed to represent the actual structure.

[0041] This minimization procedure can be carried out with aconventional least squares fitting routine such as a Levenberg-Marquardtalgorithm. It would also be possible to use a genetic algorithm. (See,U.S. Pat. No. 5,953,446.)

[0042] Ideally, the minimization routine will be carried out in realtime, associated with the measurements. Since the calculations relatedto this analysis are very complex, real time calculations can be achallenge. Some approaches for dealing with complex real timecalculations are set forth in our co-pending U.S. patent applicationSer. No. 09/906,290, filed Jul. 16, 2001, incorporated herein byreference.

[0043] Another approach to dealing with the processing difficulties isto create a library of solutions prior to the measurement. In thisapproach, a range of possible structures and their associatedtheoretical output signals are generated in advance. The results arestored as a library in a processor memory. During the measurementactivities, the actual measured signals are compared with sets oftheoretically generated output signals stored in the library. Thestructure associated with the set of theoretical signals which mostclosely matches the actual measured data is assumed to most closelyrepresent the geometry of the measured structure. The use of librariesis disclosed in U.S. Patent Application 2002/0035455 A1. Still anotherapproach is to create a much smaller database of possible solutions.Measured data can then be compared to the database and algorithms areused to interpolate between data points to derive a solution to thegeometry of the structure. (See for example, U.S. Patent Application2002/0038196 A1)

[0044] Theory

[0045] As noted above, an isolated feature should not be modeled in thesame manner as a diffraction grating. We have addressed the issue withtwo different approaches. The first approach uses Fourier expansionswhich are analogous in many ways to the Rigorous Coupled Wave Theory. Wehave also developed an analysis using a boundary integral approach.Previously we have developed a boundary integral approach for periodicgratings using Green's functions. This work is described in U.S. patentapplication Ser. No. 10/212385, filed Aug. 5, 2002 and incorporatedherein by reference.

[0046] We have found that the analysis by Fourier expansion tends to befaster than the boundary integral approach. Fourier expansion isrelatively easy to implement since it is similar to the periodic case.However, the Fourier expansion approach is less stable. Further, thereare more control parameters one has to adjust, including the number ofintegration points in the exterior and interior regions, and the cutoffintegration parameter s₀. The boundary integral approach is morenumerically involved and is more difficult to implement. However, it ismore robust. We have used both approaches to evaluate isolated lineswith single material in the line. The results of both approaches agreevery well, indicating that our results are appropriate.

[0047] Fourier Expansions

[0048] An isolated or aperiodic feature (e.g. single line or trench) canbe viewed as an array of lines or trenches with very large periods.Thus, in a approach which is analogous to the RCWA theory of thediffraction gratings, we can write the electric field as $\begin{matrix}{{E( {x,z} )} = {\int{{k}\quad {E( {k,z} )}^{\quad k\quad x}}}} \\{= {k_{0}{\int{{s}\quad {E( {{\sin \quad \theta},z} )}\quad ^{\quad k_{0}s\quad x}}}}} \\{\approx {k_{0}{\sum\limits_{j}{w_{j}{f( {s_{j},z} )}^{\quad k_{0}s_{j}x}}}}}\end{matrix}$

[0049] where s is the equivalent of sin θ, the s_(j) are the Guassianquadrature nodes, in contrast to the periodic systems where s_(j) areequally spaced, the w_(j) are the weights. The input electric field iswritten as

E ₀ =k ₀∫_(s) ₀ ^(s) ^(₀) dsE ₀(s, z)e ^(ik) ^(₀) ^(sx)

[0050] where s₀≦1. In general, the electric field as a function of s arenot analytic, we need to perform the integration in at least 3 regions,(−∞,−1),(−1,1),(1,∞). Furthermore, we need to have a cutoff s_(max) fors so that in each region we can use Gaussian quadrature for theintegrations. For TM mode, we use the equivalent of TM1 implementation.

[0051] Boundary Integral Approach

[0052] As noted above, although a Fourier expansion approach can beused, boundary or volume integral methods are ideal for isolatedfeatures in that they provide a more robust solution. The boundaryintegral formulation relies on the Green's theorem, the properties ofthe wave function and Green's functions.

[0053] For the isolated feature situation, the equation of motion forthe TE mode is

Δψ(x)−ε(x)ψ(x)=0,

ΔG(x, x′)−ε(x)G(x, x′)=δ(x−x′).

[0054] and Green's theorem states that $\begin{matrix}{{\int{{V}\quad {\psi (x)}{\delta ( {x - x^{\prime}} )}}} = {\int{{V\lbrack {{{\psi (x)}\Delta \quad {G( {x,x^{\prime}} )}} - {\Delta \quad {\psi (x)}{G( {x,x^{\prime}} )}}} \rbrack}}}} \\{= \{ \begin{matrix}{{\psi ( x^{\prime} )},} & x^{\prime} & ɛ & V \\{{\frac{1}{2}{\psi ( x^{\prime} )}},} & x^{\prime} & ɛ & \Gamma \\{0,} & x^{\prime} & & V\end{matrix} } \\{= {\int{{{\Gamma \quad\lbrack {{{\psi (x)}{\partial_{n}{G( {x,x^{\prime}} )}}} - {{\partial_{n}{\psi (x)}}{G( {x,x^{\prime}} )}}} \rbrack}}.}}}\end{matrix}$

[0055] where Γ represents the boundary of the region of interest. In ourcase, it is simply a curve since the y dimension is of no concern. Sincethe boundary conditions require that ψ(x), G(x,x′), ∂_(n)ψ(x), and∂_(n)G(x,x′) be continuous across material boundaries, the volume can beextended over several materials provided that G is obtained. For TMmode,

∇ε⁻¹∇ψ(x)−ψ(x)=0,

∇ε⁻¹ ∇G(x,x′)−G(x,x′)=δ(x−x′).

[0056] We use modified Green's theorem $\begin{matrix}{{\int{{V}\quad {\psi (x)}{\delta ( {x - x^{\prime}} )}}} = {\int{{V\lbrack {{{\psi (x)}{\nabla ɛ^{- 1}}{\nabla G}( {x,x^{\prime}} )} -} }}}} \\ {{\nabla ɛ^{- 1}}{\nabla{\psi (x)}}{G( {x,x^{\prime}} )}} \rbrack \\{= \{ \begin{matrix}{{\psi ( x^{\prime} )},} & x^{\prime} & ɛ & V \\{{\frac{1}{2}{\psi ( x^{\prime} )}},} & x^{\prime} & ɛ & \Gamma \\{0,} & x^{\prime} & & V\end{matrix} } \\{= {\int{{\Gamma}\quad {{ɛ^{- 1}\lbrack {{{\psi (x)}{\partial_{n}{G( {x,x^{\prime}} )}}} - {{\partial_{n}{\psi (x)}}{G( {x,x^{\prime}} )}}} \rbrack}.}}}}\end{matrix}$

[0057] Since the boundary conditions are ψ(x), G(x,x′), ε⁻¹∂_(n)ψ(x),and ε⁻¹∂_(n)G(x,x′) and are continuous, the integration is againmeaningful over different material domains. We write the total field inthe exterior region as a combination of incident field and scatteredfield

ψ(X)=ψ₀(X)+ψ_(s)(X),

[0058] If X′ is in the exterior region, we have $\begin{matrix}{{\int{{\Gamma}\quad {n \cdot \lbrack {{{\psi (X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G( {X,X^{\prime}} )}}} \rbrack}}} =} \\{{\int{{\Gamma_{\infty}}{n \cdot \lbrack {{{\psi (X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G( {X,X^{\prime}} )}}} \rbrack}}} +} \\{{\int{{\Gamma_{0}}{n \cdot \lbrack {{{\psi (X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G( {X,X^{\prime}} )}}} \rbrack}}} =} \\{{\int{{\Gamma_{\infty}}{n \cdot \lbrack {{{\psi_{0}(X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi_{0}(X)}}{G( {X,X^{\prime}} )}}} \rbrack}}} +} \\{{\int{{\Gamma_{0}}{n \cdot \lbrack {{{\psi_{s}(X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi_{s}(X)}}{G( {X,X^{\prime}} )}}} \rbrack}}} =} \\{{\psi_{0}( X^{\prime} )} + {\int{{\Gamma_{0}}{n \cdot \lbrack {{{\psi_{s}(X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi_{s}(X)}}{G( {X,X^{\prime}} )}}} \rbrack}}}}\end{matrix}$

[0059] When X′ is on the boundary Γ₀, we have $\begin{matrix}{{\int{{\Gamma}\quad {n \cdot \lbrack {{{\psi (X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G( {X,X^{\prime}} )}}} \rbrack}}} =} \\{{\psi_{0}( X^{\prime} )} + {\int{{\Gamma_{0}}{n \cdot \lbrack {{{\psi (X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G( {X,X^{\prime}} )}}} \rbrack}}}} \\{{\frac{1}{2}{\psi ( X^{\prime} )}} = {{\psi_{0}( X^{\prime} )} - {\int{{\Gamma_{0}^{\prime}}{n \cdot \lbrack {{{\psi (X)}{\nabla{G( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G( {X,X^{\prime}} )}}} \rbrack}}}}}\end{matrix}$

[0060] While in interior region, we have${\frac{1}{2}{\psi ( X^{\prime} )}} = {\int{{\Gamma_{0}^{\prime}}{n \cdot \lbrack {{{\psi (X)}{\nabla{G^{\prime}( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G^{\prime}( {X,X^{\prime}} )}}} \rbrack}}}$

[0061] Combine the two we have $\begin{matrix}{{{\frac{1}{2}{\psi ( X^{\prime} )}} + {\int{{\Gamma_{0}^{\prime}}{n \cdot \lbrack {{{\psi (X)}{\nabla{G_{ext}( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G_{ext}( {X,X^{\prime}} )}}} \rbrack}}}} = {\psi_{0}( X^{\prime} )}} \\{{{\frac{1}{2}{\psi ( X^{\prime} )}} - {\int{{\Gamma_{0}^{\prime}}{n \cdot \lbrack {{{\psi (X)}{\nabla{G_{int}( {X,X^{\prime}} )}}} - {{\nabla{\psi (X)}}{G_{int}( {X,X^{\prime}} )}}} \rbrack}}}} = 0}\end{matrix}$

[0062] When these last two equations are discretized either withboundary element or quadrature method, they become a set of linearequations which can be solved directly if the dimensions are relativelysmall or iteratively if the system is large.

[0063] We have performed initial testing using the subject method tomeasure an isolated trench formed on a semiconductor wafer. The waferwas silicon with a 650 nm top layer of silicon dioxide. The isolatedtrench had a nominal width of about 500 nanometers (0.5 microns) and anominal depth of about 650 nanometers. A beam profile reflectometer ofthe type illustrated in FIG. 1 and found in the assignee's Opti-Probesystem was used to measure the sample. The photodetector arrays in thissystem generate output signals corresponding to angles of incidenceranging from about +70 degrees to −70 degrees around the normal to thesample.

[0064]FIGS. 2 and 3 illustrate measurements made on the sample wafer.FIG. 2 illustrates measurements with the probe beam over the trenchwhile FIG. 3 illustrates measurements with the probe beam positioned onan unpatterned portion of the wafer. In each Figure, the circlesrepresent actual measurements plotted as a function of angle ofincidence. The smooth lines represent a theoretical fit of the data.

[0065] The differences between the data in FIGS. 2 and 3 illustrate thata single narrow trench in the field of view effects the BPR measurement.The parameters used to create the solid line in FIG. 2 are based on atrench having dimensions illustrated in FIG. 4. These initialexperiments fully demonstrate the feasibility of using BPR to measure asmall, isolated, aperiodic feature on a semiconductor sample.

[0066] The subject invention is not limited specifically to the BPRarrangement illustrated above. For example, various other forms ofdetector arrays can be used. In particular, it is feasible to replaceone or both of the arrays with a two dimensional array such as a CCD.Those skilled in the art will readily be able to envision othermodifications, including those described in the patent documents citedherein.

[0067] As noted above, the output of the detector elements includes botha specularly reflected component and a scattered light component. It maybe desirable to minimize the specularly reflected component and maximizethe scattered light component. This can be important if the primaryinterest is in characterizing the physical structure of the scattererand less information is needed about the underlying structure.

[0068] One approach for achieving the latter goal is described incopending provisional application Serial No. 60/394,201 filed Jul. 5,2002, assigned to the same assignee herein and incorporated byreference. This disclosure proposes inserting a baffle 80 (shown inphantom in FIG. 1) into a portion of the probe beam path. The baffle isintended to block a semi-circular portion of the probe beam. In thisarrangement, light in the left hand portion of the incident beam reachesthe sample while light in the right hand portion of the incident beam isblocked. Specularly reflected light is collected by the right hand sideof lens 40 but is blocked from reaching the detector by baffle 80. Incontrast, only light which has been scattered from the sample andcaptured by the left hand side of the lens will reach the detector. Inthis manner, the light reaching the detector will be primarily scatteredlight and not specularly reflected light. Additional measurements can betaken after moving the baffle to block the left hand side of theincident beam.

[0069] As can be appreciated, with a baffle in place, the outputgenerated by the detectors does not correspond to specific incidenceangles as in a conventional BPR arrangement. Rather, the detectorsmeasure an integration of scattered light from various angles ofincidence.

[0070] A measurement system including a baffle in the manner describeabove can be used to measure scattered light from both isolatedstructures as well as periodic structures. Although the use of a baffleis not presently claimed in this application, it is intended that thescope of the claims include such an embodiment.

[0071] The subject method is not limited to reflectometry. As noted inU.S. Pat. Nos. 5,042,951 and 5,166,752 (incorporated herein byreference), it is also possible to obtain ellipsometric measurementscorresponding to ψ and Δ simultaneously at multiple angles of incidence.To obtain such measurements, some additional optical elements should beadded to the device of FIG. 1. For example, a polarizer 66 (shown inphantom) is desirable to accurately predetermine the polarization stateof the probe beam. On the detection side, an analyzer 68 (also shown inphantom) is provided to aid in analyzing the change in polarizationstate of the probe beam due to interaction with the sample. The opticalcomponents of the analyzer can be of any type typically used in anellipsometer such as a polarizer or a retarder. The ellipsometric outputsignals are analyzed in a fashion similar to the prior art approachesfor using ellipsometric data to evaluate the geometry of an aperiodicstructures.

[0072] U.S. Pat. No. 5,181,080 describes a variant of the BPE approach.In this system, a quadrant detector is used to measure the power of thereflected probe beam along two orthogonal axes. Each quadrant generatesa response proportional to the integration of all angles of incidence.By manipulating the output of the quadrants, ellipsometric informationcan be derived.

[0073] It is also within the scope of the subject invention to combinethe BPR and BPE measurements with other measurements that might beavailable from a composite tool. As noted above, the assignee'sOpti-Probe device has multiple measurement technologies in addition tothe Beam Profile Reflectometry system. These other technologies includebroadband reflectometry and broadband ellipsometry. In these measurementtechnologies, an incoherent polychromatic probe beam is focused onto thesample. The reflected polychromatic probe beam is monitored at aplurality of wavelengths. In reflectometry, changes in intensity of thepolychromatic probe beam are monitored. In ellipsometry, changes inpolarization state of the polychromatic probe beam are monitored. Theoutput from these additional modules might be useable in combinationwith the BPR and BPE signals to more accurately evaluate the geometry ofthe isolated structure.

[0074] As can be appreciated, most of the basic hardware elementsdiscussed herein have been known in the prior art. The developmentsintended to be covered by this disclosure relate to certain applicationsof that technology. More specifically, it is believed that the BPR andBPE techniques can be used to measure isolated, single (non-repeating)structures which have dimensions in the micron and sub-micron range.These would include, for example, single lines, single vias, singleholes and single trenches. It is believed that by focusing a coherentprobe beam on the structure and measuring the reflected response, onecan rely on scattered light effects to determine the geometry of thestructure.

[0075] It may also be possible to evaluate a single structure that waslarger than the probe beam spot (i.e. was only partially illuminated).By measuring scattered light effects, and with some a priori knowledgeof the structure, one may be able to determine the geometry of thestructure which is being illuminated.

[0076] The subject invention is also intended to cover the situationwhere a periodic structure has a size or extent smaller than the probebeam spot. For example, consider a repeating structure having only fiveor ten lines. If the probe beam spot is greater than the structure, thereflected field would be aperiodic and could not be analyzed with agrating approach. However, using the techniques described herein onecould still derive information about the sample. In this case, it isassumed that the probe beam spot size would be at least about twice thewidth of the structure so the grating effects would be minimal.

[0077] Although initial experiments have shown that isolated andaperiodic structures can be monitored with the probe beam spatiallyfixed with respect to the feature, it is within the scope of the subjectinvention to scan the probe beam with respect to the feature. In such acase, measurements are taken at various positions of the probe beam andthe data combined to analyze the sample. Such an approach may beparticularly useful with a system operating in accordance with U.S. Pat.No. 5,181,080, discussed above. The relative motion of the probe beamwith respect to the sample can be achieved using a conventional motionstage. It is also possible to move the optics. (See for example, PCT WO00/57127.)

[0078] While the subject invention has been described with reference toa preferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

We claim:
 1. A method for monitoring the process of fabricating anaperiodic structure on semiconductor wafers comprising the steps of:generating a probe beam of radiation; focusing the probe beamsubstantially normal onto the aperiodic structure to create a spread ofangles of incidence that extends from the normal to at least sixtydegrees; monitoring the light from the probe beam diffracted from thestructure with an array of photodetecting elements and simultaneouslygenerating a plurality of independent output signals corresponding to arange of angles of incidence from the normal of sixty degrees; andcomparing the output signals with a set of expected signalscorresponding to the desired geometry of the aperiodic structure todetermine if the process is falling within specified tolerances.
 2. Amethod as recited in claim 1, wherein the structure is an isolatedfeature.
 3. A method as recited in claim 1, wherein the structure is anisolated feature selected from the group consisting of a line, a hole, avia, a trench and a overlay registration marking.
 4. A method as recitedin claim 1, wherein the probe beam is focused to spot less than fivemicrons in diameter.
 5. A method as recited in claim 1, wherein theprobe beam is focused to spot less than two microns in diameter.
 6. Amethod as recited in claim 1, wherein the intensity of the probe beamlight is monitored.
 7. A method as recited in claim 1, wherein thechange in polarization state of the probe beam light is monitored.
 8. Amethod as recited in claim 1, wherein probe beam light focused on thesample is coherent.
 9. A method as recited in claim 1, wherein the probebeam is scanned with respect to the structure being evaluated.
 10. Amethod as recited in claim 1, further including the steps of:illuminating the structure with an incoherent polychromatic probe beam;monitoring the reflected polychromatic probe beam; generating secondoutput signals corresponding to a plurality of wavelengths; comparingthe second output signals with a second set of expected signalscorresponding to the desired geometry of the aperiodic structure todetermine if the process is falling within specified tolerances.
 11. Amethod as recited in claim 10, wherein changes in intensity of thepolychromatic probe beam are monitored.
 12. A method as recited in claim11, wherein changes in polarization state of the polychromatic probebeam are monitored.
 13. A method as recited in claim 1, wherein theprobe beam is monitored along two orthogonal axes.
 14. A method asrecited in claim 1, wherein the probe beam is monitored using a twodimensional array of detectors.